Field of the Invention
This invention relates to earthboring, in general, and specifically to constant velocity universal joints. Universal joints have general utility in the realm of power transmission as a mechanism for redirecting an axial line of rotary torque. The present invention has particular value to the industrial art of directional well drilling.
Discussion of Prior Art
One procedure for directional drilling of boreholes in the earth includes a downhole drilling motor (also called a mud motor) for rotatively driving the drill bit. Drilling motors are modern adaptions of the ancient Archimedes screw used for lifting or pumping water but is operated in reverse. To drill directionally, drilling fluid essential for rotary drilling is pumped down the central bore of a pipe string. Just prior to reaching the drill bit, the drilling fluid is directed through the drilling motor. At the uphole end of the drilling motor the Archimedes screw is used to convert fluid energy into rotating mechanical energy. The drilling fluid acts against a helically lobed shaft, known as a rotor, which rotates about its axis within a correspondingly lobed housing known as a stator. The stator along with the drill string above and drilling motor outer housings below remain stationary. Only the rotor, output drive shaft and drill bit rotate when drilling in this mode.
To directionally drill or generate arced curvature of the wellbore, the rotary drive axis of the drill bit must be deviated from the uphole axis of the drilling motor. The traditional means for changing the angular direction of the motor output drive shaft is with a mechanism characterized as a universal joint. All universal joints must transmit both compressive and torque load from the rotor/stator power section to the bearing assembly. One of the most popular universal joint mechanisms favored by the earthboring industry is that described as a “constant velocity” or CV joint.
Generally, CV joints comprise mirrored upper and lower ball and socket housing arrangements. To accommodate axial compression loads, most employ some method of spherical ball bearing or semi-spherical ball shape secured to the end of a drive shaft which fits within a socket housing having a mating, semi-spherical pocket.
To transmit torque loads, the CV joint ball and socket housing are mechanically linked by a plurality of torque transfer balls. The drive shaft ball, hereafter characterized as the “ball”, typically confines the torque transfer balls within mating cups. The cups are angularly spaced equally about the ball perimeter in the diametric plane transverse to the in-drive axis. The cup diameters are substantially the same as the torque transfer balls but less than half the hemisphere depth.
The socket housing pocket, hereafter characterized as the housing “socket”, typically contains a plurality of race channels parallel with the socket out-drive axis distributed about the internal surface of the pocket. The number of race channels must exactly match the circumferential location of the mating driveshaft ball cups and are angularly spaced equally about the pocket perimeter. The channel depth is less than a hemisphere of each ball.
As the in-drive and out-drive shafts rotate, torque transfer balls shift along the socket channels from one side of a transverse diameter plane to the other. The torque force is transferred through the torque transfer balls from the socket surface area to an axially moving arced line across a respective channel. Consequently, most of the mechanical wear on the joint occurs to the socket channels in the area of the shifting ball contact line.